1. Field of the Invention
The present invention relates to an X-ray computed tomography apparatus and a tomographic method.
2. Description of the Related Art
Conventionally, an X-ray Computed Tomography apparatus (hereinafter, “X-ray CT apparatus”) plays an important role in various medical practices, such as a diagnosis of a disease, a treatment, and an operation plan, by reconstructing and providing a tomographic image that indicates morphologic information about human body tissue in a subject body on a plane (fault plane) irradiated with an X-ray based on X-ray intensity distribution data that is obtained by detecting the intensity of an X-ray passed through the subject.
Because a heart is an organ that beats constantly, improvement in a temporal resolution of a heart tomographic image is important for a cardiac examination through which a heart disease is diagnosed by using a heart tomographic image of a subject provided by an X-ray CT apparatus. To reconstruct a heart tomographic image, generally performed is an electrocardiogram-gated reconstruction method of reconstructing a heart tomographic image in a high temporal resolution by processing X-ray intensity distribution data of a plurality of heartbeats collected in a period during which a heart action is relatively less active by using an electrocardiographic waveform of a subject acquired from an electrocardiograph attached to the subject.
As an electrocardiogram-gated reconstruction method, there are a Retrospective Gating method (hereinafter, “RG method”), and a Prospective Gating method (hereinafter, “PG method”). According to the RG method, X-ray intensity distribution data of a plurality of heartbeats are continuously collected by continuously irradiating a subject with X-rays. Each piece of X-ray intensity distribution data in a specific phase, for example, a phase in the vicinity of a U-wave that indicates a small action of the heart, is extracted from each piece of the continuously collected X-ray intensity distribution data of the heartbeats, based on an electrocardiographic waveform of the subject acquired simultaneously from the electrocardiograph. Accordingly, a heart tomographic image in a high temporal resolution is reconstructed. According to the PG method, based on an electrocardiographic waveform of a subject preliminarily acquired from an electrocardiograph, for example, X-rays are intermittently irradiated only in a specific phase in the vicinity of a U-wave that indicates a small action of the heart, or the intensity of X-rays to be irradiated is modulated to a high intensity in the specific phase, accordingly, X-ray intensity distribution data of a plurality of heartbeats in the specific phase are collected, and then a heart tomographic image in a high temporal resolution is reconstructed.
According to any of the RG method and the PG method, a scanning schedule of a heart tomographic image needs to be determined in advance based on an electrocardiographic waveform (for example, an average interval between R-waves) of the subject acquired from the electrocardiograph.
For example, according to the RG method, when collecting X-ray intensity distribution data of two beats, an X-ray irradiation start time is calculated from an average interval time between R-waves preliminarily acquired from an electrocardiograph attached to the subject, and furthermore an X-ray irradiation end time is calculated by estimating a time required for the heart of the subject to beat twice based on the preliminarily acquired average interval time between the R-waves, so that a scanning schedule of a heart tomographic image is determined. An X-ray irradiation is then started in accordance with the determined scanning schedule.
If a heartbeat abnormality, such as arrhythmia caused by extrasystole or a heartbeat fluctuation, occurs, there is a possibility that sufficient X-ray intensity distribution data required for reconstructing a heart tomographic image are not obtained by scanning the heart even in accordance with the scanning schedule. In other words, as shown in FIG. 9A, when intervals between R-waves after the start of scanning indicate a sinus rhythm (normal heartbeat) that intervals are insignificantly different from the preliminarily acquired average interval time between R-waves, data of two normal heartbeats can be acquired on schedule by performing X-ray irradiation in accordance with the determined scanning schedule, and a heart tomographic image can be reconstructed in an expected temporal resolution. However, as shown in FIG. 9B, when an interval between R-waves changes substantially during X-ray irradiation due to an abnormal heartbeat, there is a problem that even if the X-ray irradiation is performed in accordance with the determined scanning schedule, intended X-ray intensity distribution data of two normal heartbeats cannot be obtained, consequently an image cannot be reconstructed in the expected temporal resolution. FIGS. 9A to 9D are schematic diagrams for explaining a conventional technology.
JP-A 2005-66042 (KOKAI) discloses a technology for acquiring a required amount of intended X-ray intensity distribution data in a specific phase while coping with an irregular heartbeat after the start of scanning by monitoring data acquired from an electrocardiograph also during the scanning. Specifically, while irradiating a subject with X-rays, intervals between R-waves are monitored by using data acquired from the electrocardiograph, and for example, as shown in FIG. 9C, if an abnormal heartbeat is detected while scanning, data of two normal heartbeats are acquired by changing the scanning schedule and extending a period for irradiating the subject with X-rays.
According to the conventional technology, there is a problem that the scanning schedule cannot be changed in response to an abnormal heartbeat occurring before the scanning, because data acquired from the electrocardiograph is not monitored after the scanning schedule is determined until actually starting the scanning, as a result, an X-ray dosage tends to be large to acquire a required amount of intended X-ray intensity distribution data in a specific phase.
For example, as shown in FIG. 9D, if an abnormal heartbeat occurs before scanning, an actual electrocardiographic waveform deviates from an expected electrocardiographic waveform in a starting period of X-ray irradiation in the determined scanning schedule, as a result, an X-ray dosage tends to be large to acquire intended X-ray intensity distribution data of two normal heartbeats in a specific phase, which is a problem. In other words, the conventional technology has a problem that even if an abnormal heartbeat occurs before scanning, a required amount of intended X-ray intensity distribution data in a specific phase can be acquired; however, the scanning schedule cannot be changed in response to the abnormal heartbeat occurring before the scanning, thereby causing an unnecessary exposure to X-rays.
As described above, the Retrospective Gating method has the problem that an X-ray dosage tends to be large to acquire a required amount of intended X-ray intensity distribution data in a specific phase, and also the Prospective Gating method has a similar problem.